Magnetic resonance gradient coil and RF screen

Abstract: The magnetic field assembly of a magnetic resonance imaging device includes an annular superconducting magnet (10) which is mounted within a toroidal vacuum vessel (24). A cylindrical member (26) defines a central bore through which the superconducting magnets generate a temporally constant primary magnetic field. A cylindrical, dielectric former (46) is mounted in the bore displaced a small distance from the cylindrical member. A radio frequency coil (32) is mounted within the cylindrical member defining a patient receiving examination region. An RF shield (34) is mounted around and exterior peripheral surface of the former. Primary gradient coils (40) are mounted around and potted to the exterior of the dielectric former around the RF shield. Gradient shield or secondary coils (44) are potted around an exterior of the cylindrical member within the vacuum chamber. As illustrated in FIGURE 3, when unshielded gradient coils are used, the primary gradient coils and the RF shield are mounted around the outer diameter of the cylindrical member (26).

Description: This invention relates to a magnetic resonance gradient coil and RF screen. It finds particular application in conjunction with self-shielded gradient and RF coil assemblies for magnetic resonance imaging apparatus and will be described with particular reference thereto. However, it will be appreciated that the present invention will also find application in conjunction with magnetic resonance spectroscopy systems and other applications which require RF fields and gradient magnetic fields. Heretofore, magnetic resonance imagers have utilized a cylindrical superconducting magnet assembly which generates a temporally constant primary magnetic field axially through a room temperature bore or central cylinder of the magnet assembly. The magnet windings are maintained at liquid helium temperatures in a cryostat. This structure is double walled to provide a vacuum space, intermediate cold shields, and room for superinsulation. Cryostat construction is optimized to minimize helium boil-off. The room temperature bore of the magnet provides space for gradient magnetic field coils, RF coils, and, of course, the object or person to be imaged. The gradient coils generate axial fields which have gradients in three mutually orthogonal directions, x, y, and the axial direction z. Current waveforms are applied to the gradients during the MR process. The gradients in turn induce eddy currents in the metal structures of the magnet cryostat. The magnetic fields of the eddy currents then become part of the magnetic fields seen by the object being imaged. To the extent that the eddy currents mimic the gradient that caused them, they could be corrected by pre-emphasis. In general, it helps to maintain some spacing between the gradient and magnet structures that might sustain eddy currents. More recently, self shielded gradients have been developed that eliminate the eddy currents in the magnet structure (GB-A-2 259 773, WO 91/19209). Self shielded gradients consist of a primary set of coils on a cylindrical former and a shield set of coils on a second former at a larger radius. At radii outside the shielded gradient, the magnetic fields produced by the primary coils are substantially cancelled by those of the shielding coils. The gradient fields of both sets of coils combine to produce axial directed fields with substantially first order intensity gradients in the x, y, and z-directions. It is desirable that the primary and shield coils have a radius ratio of 1.3 or more so the shield coil does not cancel too much of the field of the primary within the imaging volume. With self shielded gradient coils, the distance from the primary gradient coil to the magnet structures is not critical. The overall efficiency or power dissipated by the gradient coil is strongly dependent on the spacing between the primary gradient coil and the shield gradient coil. Efficiency is optimized, i.e. power consumption is minimized, when the diameter of the primary gradient field coil is minimized and the diameter of the shield coil is maximized. However, 100 cm. bore magnets in general cost more than smaller, e.g. 90 cm. bore magnets because of materials costs. In order to reduce magnet cost, one would like to reduce the bore diameter consistent with homogeneity requirements. Likewise, in order to improve the gradient efficiency and get more gradient per imposed ampere, one would like to use the maximum space available for the outer coils of a shielded gradient. The maximum diameter of the shield gradient coil assembly is limited by the interior bore of the superconducting magnet or, more commonly, by a shimset of ferrous metal shims mounted within the bore for improving the homogeneity of the main magnetic field. An RF body coil which is inserted inside the gradient coil generally had an inner diameter of 55-60 cm. RF screens are placed at some radius beyond the RF coil with the specific purpose of generating a known eddy current pattern that is not RF lossy. In addition, there is a degree of isolation between the RF and the gradient coils. For example, the RF screen has been applied to the inner surface of the gradient coil former. The screen is made of overlapping strips of adhesive backed copper about 2-3 inches wide completely covering the surface. The overlap region serves to capacitively couple the strips at 21 to 64 MHz so they look like a reasonably conductive surface. If the adhesive on the copper is RF lossy, it will reduce the Q-factor of the RF coil and thus the signal to noise ratio for NMR. The copper is in strips to reduce the gradient eddy currents generated therein. One of the important design considerations of the RF screen is the distance between the RF coil conductive elements and the screen. Eddy currents in the screen also generate fields that oppose the fields of the RF coil reducing the overall field. In a sense these "image currents" mimic, in a functional sense, the shield coils of the gradient coil. The screen must produce predictable, reproducible results without RF losses. Beyond the RF screen there is substantially zero RF field. One disadvantage of the use of adhesive backed copper in the RF coil shield is that it is labor-intensive. Typically, on the order of 32 or more segments are required around the periphery of the inner diameter of the gradient coil bore. A further disadvantage is that the adhesive can be RF lossy and lower the achievable signal-to-noise ratio. Another method for constructing the RF shield is to use a printed circuit approach with a low-loss substrate. One disadvantage of the printed circuit board approach is its cost. Substrate materials have been used to obtain the desired low loss characteristics and provide a means for mounting the screen to the gradient inner diameter. This increases processing and fabrication costs as well. A third and more cost-effective method is to use a fine copper mesh, much like window screen. The broken pattern of the woven mesh is intrinsically worse at conducting low frequency eddy currents than solid copper. One disadvantage to the use of RF mesh is that mounting the mesh to the interior diameter of the gradient coil is difficult. The screen must be mounted securely with good dimensional control. Generally, an additional substrate is required to support the mesh. This increases the manufacturing costs and consumes valuable space. Thus, the prior art shielded RF and gradient coils assemblies are thicker in radial dimension. Such thicker RF and gradient coils can be utilized and still maintain the conventional inner diameter of the bore if large diameter, e.g. 100 cm. bore superconducting magnets are utilized. However, the cost of the superconducting magnet varies with its diameter. Accordingly, it is generally considered desirable to reduce the diameter of the inner bore of the magnet, not enlarge it. Reducing the spacing between the primary and shield gradient coils would reduce efficiency and create significant heating problems. That is, much of the energy consumed by the gradient coil is turned into heat. In an inefficient system in which a large amount of heat is generated, this creates problems in dissipating the heat. Similarly, even a small increase in the relative diameters of the primary RF and shield coils can have a significant change in the amount of energy consumed. For example, a 2% increase in the diameter of the gradient shield coil can result in a 20% decrease in heat energy dissipation. In accordance with one aspect of the present invention, the RF screen is supported by the outer surface of the same cylindrical member as supports the primary gradient coil assembly. This permits the gap between the RF body coil and the RF shield to be maximised, with a consequent increase in signal-to-noise ratio and decrease in RF transmit power. According to a preferred feature of the invention, a gradient coil assembly is supported by an outer surface of a cylindrical member of a vacuum chamber of the superconducting magnet, i.e. within the vacuum chamber of the superconducting magnet assembly. In the case of a self-shielded gradient coil, this allows the efficiency of the coil to be improved by permitting an increase in the spacing between the primary and shield gradient coils. An advantage of an apparatus embodying the present invention is that it maximizes the internal usable bore of an MRI magnet while minimizing the inner diameter of the superconducting magnet. Another advantage of an apparatus embodying the present invention resides in the simplicity of manufacture. The invention will now be described, by way of example, with reference to the accompanying drawings, in which:- FIGURE 1 is a diagrammatic illustration of a magnetic resonance imaging system in accordance with the present invention; FIGURE 2 is an enlarged, detailed cross-sectional view illustrating the relationship of the RF coil and RF shield, the gradient primary and shield coil, and the superconducting magnet vacuum vessel and cold shields; and FIGURE 3 is an alternative embodiment in which the primary gradient coil assembly is located within the vacuum vessel of a superconducting magnet. With reference to FIGURE 1, a plurality of superconducting magnetic field coils 10 and 18 generate a temporally constant magnetic field. Within an imaging region of a central bore 12, this field is substantially uniform and axial along a longitudinal or z-axis. Additionally, x and y-axes are usually assigned to horizontal and vertical directions to form a mutually orthogonal set. The inner superconducting magnetic field coils 10 of the superconducting magnet are supported by a former 14 which may be a part of a toroidal helium vessel or can 16. The helium vessel is filled with liquid helium to maintain the superconducting magnet coils cold enough to be superconducting with the impressed magnetic field. If required, shield coils 18 can be incorporated to limit the combined magnetic fields of coils 10 and 18 in regions surrounding the superconducting magnet. To reduce helium boil-off, the toroidal helium vessel 16 is surrounded by a first cold shield 20 which is maintained at about 20° K. A second cold shield assembly 22 is chilled to about 70° K. The first and second cold shields are preferably cooled by mechanical refrigeration (not shown). A toroidal vacuum vessel 24 encases the cold shields to define a vacuum reservoir therearound. The vacuum vessel 24 includes a cylindrical, dielectric member 26 that defines the bore 12 and extends parallel to the z-axis. The main magnetic field is generated through the cylindrical member 26. Preferably, layers of mylar insulation are arranged between the vacuum vessel and the cold shields. With continuing reference to FIGURE 1 and further reference to FIGURE 2, an RF coil assembly 30 is mounted in the bore 12. More specifically, the RF coil assembly includes an RF coil 32 and an RF shield or screen 34. The primary RF coil 32 , preferably a quadrature or birdcage type coil, has an inner diameter which defines a patient receiving bore, preferably of at least 55 cm. A self shielded gradient coil assembly 40 includes a primary whole body gradient coil assembly 42 and a secondary coil assembly 44. The primary gradient coil assembly 42 includes a cylindrical dielectric former 46. Preferably, the former has a sufficient thickness to maintain its structural integrity under the forces associated with generating the magnetic field gradients within the large constant magnetic field of superconducting coils 10 and 18, e.g. 5-10 mm of glass reinforced plastic, preferably about 7.5 mm. The former 46 has a series of grooves 48 defined or cut in its outer surface in accordance with the pattern of a z-gradient coil 50 . The RF shield 34, preferably overlapped and insulated strips of copper coil or a fine copper mesh, is applied to the outer surface of the former 46 and through the z-coil receiving grooves 48. Other RF shields 34 are also contemplated, such as a thin electrically conductive layer that is electro-deposited or sputtered. The RF shield mesh may be deformed into the grooves by tightly wound and insulated z-gradient wire windings, may be fitted into the grooves, may have solder connected or lapped segments extending through the grooves, or the like. The grooves 48 have a depth which generally corresponds to the thickness of the z-gradient coils such that after the z-gradient coils are inserted, a substantially circularly cylindrical surface is presented for receiving x and y-gradient coils 52,54. If the z-gradient coil is wire wound or otherwise has a construction which does not support significant eddy currents at the RF frequency, the RF shield may be outside of all or part of the z-gradient coil. The x and y-gradient coils 52, 54 are preferably fingerprint type coils as illustrated in greater detail in European Patent Application Publication No. 0587423. Due to the circular symmetry, the x and y-gradient coils are substantially identical, but offset 90° relative to each other. The x and y-gradient coils, the RF shield, and the z-gradient coils are potted in an epoxy or other dielectric material. The woven screen RF shield and the fingerprint type coils are advantageous in the potting process because they provide paths for the potting material to flow. Various alternate constructions are also contemplated. First, grooves need not be provided for the z-gradient coil. Rather, the z-gradient coil 50 may be laminated in a layer on top of the RF shield 34. It is also contemplated that fingerprint, or alternate, x/y coils are layed over the RF screen first and the z coil built last. Second, if grooves 48 are provided for the z-gradient coil, strips of foil or other RF shield material may be laid in the grooves and a continuous sheet or strips of the RF mesh or other RF shield may be wound over the z-coil. Solder edge connections, capacitive overlap connections, or the like provide electrical continuity between the strips inserted under the z-gradient coil and the remainder of the RF shield. Third, the insulated z-gradient coil may be individual windings, a cluster or bunched coil, or a distributed coil. Fourth, by winding the z-gradient coil under tension, the RF shield screen can be deformed into the grooves. A plurality of plastic I-beams or other mechanical structures 56 mount the gradient former 46 to the interior surface of the cylindrical member 26 of the superconducting magnet. The I-beams or other mechanical supports 56 define a sufficient gap for receiving main magnet field shims (not shown), typically a multiplicity of ferrous elements, for shimming the uniformity of the primary magnetic field. The secondary gradient coil 44 includes the cylindrical dielectric member 26 which preferably has grooves 60 for receiving a z-gradient shield coil 62. X and y-gradient shield or secondary coils 64, 66, are mounted on the outer circular periphery of the former 26 and potted in an epoxy potting 68. The design of the x, y, and z-gradient and gradient shield coils is preferably of the design shown in the above-referenced European Patent Application Publication No. 0587423. The primary and secondary gradient coils are coupled to create accurate gradient magnetic fields in the bore 12 and cancelling magnetic fields outside the bore, particularly in the vacuum vessel. Preferably, an annulus of more than 33 mm is provided between the primary gradient coil former 46 and the cylindrical member 26. This provides passages for air cooling, displaces the primary and shield gradient coils a maximal amount, and provides a compartment within which to insert magnetic field shims for shimming the uniformity of the main magnetic field. Preferably, the cylindrical member 26 is connected with other portions of the vacuum vessel 24 with an O-ring joint 70 to maintain the vacuum while providing a solid construction which maximizes the space available for cold shield assemblies therein. Various alternate embodiments are also contemplated. First, the primary z-gradient coil 50 may be a distributed coil defined by copper or other electrically conductive tubes. By running a cooling fluid through the tubes, greater cooling efficiency is achieved and higher power levels or gradient strengths can be run. Preferably, the cold shield 22 includes a slotted tube cylindrical cold shield portion 72 which is mounted on a cylindrical dielectric former 74. A sequence control means 80 controls a gradient coil control means 82 and a transmitter 84 . The gradient coil control 82 is connected with a series of current pulse generators 86 which, in turn, are connected with the primary gradient coils 50 , 52 , 54 and the secondary gradient coils 62 , 64 , 66 . The transmitter 84 , preferably a digital transmitter, is connected with the primary radio frequency coil for generating pulses of radio frequency signals for exciting and manipulating magnetic resonance in selected dipoles of a portion of a subject within the bore. A radio frequency receiver 88 , preferably a digital receiver, is connected with the radio frequency primary coil or surface coils (not shown) for demodulating magnetic resonance signals emanating from the examined portion of the subject. An image reconstruction means 90 , such as an inverse two-dimensional Fourier transform reconstruction means, reconstructs the received magnetic resonance signals into an electronic image representation that is stored in an image memory 92 . A video processor 94 converts electronic images stored in the memory 92 into appropriate format for display on a video monitor 96 . With reference to FIGURE 3, the present invention is also applicable to unshielded gradient coils. In the unshielded gradient coil embodiment, the RF shield mesh 34 is wrapped around the outer diameter of the vacuum vessel cylindrical member 26 and has portions inserted into the grooves 60. The primary x, y, and z-gradient coils 50, 54, 56, as described above, are wrapped around the RF shield 32 and potted in epoxy. By mounting the primary x,y, and z-gradient coils in the superconducting magnet assembly, the effective bore of the magnetic resonance imaging apparatus is smaller than the inner diameter of the cylindrical 26 only by the thickness of the RF primary coil assembly 32. The cold shields 20 and 22 and the helium vessel or can 16 are constructed of materials which substantially do not support eddy currents at gradient field frequencies. The helium can is advantageously a dielectric material. The cold shields 20 and 22 may be a thermally conductive but electrically resistive material such as graphite or diamond materials. Alternately, the cold shields may be of a thermally and electrically conductive material, but be constructed of a plurality of sufficiently narrow and thin elements such that low frequency eddy currents are not supported, e.g. a copper or aluminium mesh. As yet another alternative, gradient shield coils could be supported by the cold shield 22 if increased mechanical cooling is provided to compensate for the increased thermal loading.

Claims: A magnetic resonance imaging apparatus comprising: a toroidal superconducting magnet assembly (10, 14, 16, 18, 20, 22, 24, 26) including a generally cylindrical, dielectric member (46; 26) which defines a longitudinally extending bore (12) which extends around the examination region; a gradient coil means for causing magnetic field gradients within the bore, the gradient coil means including a primary gradient coil assembly (50, 52, 54) supported by the outer surface of the cylindrical member (46; 26); an RF coil assembly (30) for transmitting radio frequency pulses into the examination region, the RF coil assembly including an RF coil (32) disposed within the bore (12) and an RF shield (34); a sequence control means for controlling the gradient coil means and the radio frequency coil assembly characterised in that the RF shield (34) is supported by the outer surface of the same cylindrical member (46; 26) as supports the primary gradient coil assembly (50, 52, 54). An apparatus according to Claim 1, wherein the RF shield (34) is disposed along the outer surface of the cylindrical member (46; 26). An apparatus according to Claim 2, wherein the primary gradient coil assembly is disposed around the RF shield. An apparatus as claimed in Claim 3, wherein the radio frequency shield (34) includes an electrically conductive mesh sandwiched between the primary gradient coil assembly and the outer surface of the cylindrical member (46; 26). An apparatus as claimed in any one of Claims 1 to 4, wherein the radio frequency shield (34) is wrapped around the cylindrical member (46; 26). An apparatus as claimed in any one of Claims 1 to 5, wherein the cylindrical dielectric member (46) has grooves (48) defined in the outer surface thereof for receiving a portion of the primary gradient coil assembly, the RF shield (34) extending through the grooves below the portion of the primary gradient coils disposed therein such that the RF shield is disposed between the primary gradient coil assembly and the RF coil assembly (30). An apparatus as claimed in any one of Claims 1 to 6, wherein the radio frequency shield (34) includes fine metal mesh. An apparatus as claimed in Claim 7, wherein the RF shield (34) and the gradient coil assembly (50, 52, 54) are potted in epoxy, the RF shield mesh layer defining epoxy flow paths which facilitate potting. An apparatus as claimed in any one of Claims 1 to 8, wherein the primary gradient coil assembly is within a vacuum chamber of the superconducting magnet assembly. An apparatus as claimed in any one of Claims 1 to 8, wherein the gradient coil means is a self-shielded gradient coil means, the self-shielded gradient coil means including: a second cylindrical member (26) surrounding and displaced from the first-mentioned cylindrical member (46) supporting the primary gradient coil assembly (50, 52, 54); a shield gradient coil assembly (62, 64, 66) disposed around the second cylindrical member (26), the primary and shield gradient coil assemblies being coupled such that within the examination region their combined fields create selected magnetic field gradients and in at least selected regions outside of the shield gradient coil assembly (62, 64, 66), their combined fields are substantially zero. An apparatus as claimed in any one of Claims 1 to 10, wherein a gradient coil assembly (62, 64, 66; 50, 52, 54) of the gradient coil means is supported by an outer surface of a cylindrical dielectric member (26) of a generally toroidal vacuum chamber (24) of the toroidal superconducting magnet assembly (10, 14, 16, 18, 20, 22, 24, 26) and within the vacuum chamber. An apparatus as claimed in Claim 11, wherein the superconducting magnet assembly includes an annular superconducting magnet (10, 18) disposed within the toroidal vacuum chamber (24), a helium vessel (16) disposed within the toroidal vacuum chamber (24) and surrounding the annular superconducting magnet for maintaining the superconducting magnet chilled to a temperature at which it is superconducting. An apparatus as claimed in Claim 12, further including at least one cold shield (20, 22) disposed between the helium vessel (16) and the toroidal vacuum chamber (24), the cold shield including a thermally conductive tube surrounding and spaced from the vacuum vessel cylindrical member, the thermally conductive tube being slotted to inhibit the generation of eddy currents therein.

Inventors: DEMEESTER GORDON D, MORICH MICHAEL A, PATRICK JOHN L, ZOU XUEMING NMI, DEMEESTER, GORDON D., MORICH, MICHAEL A., PATRICK, JOHN L., ZOU, XUEMING, (NMI)
Applicants: MARCONI MEDICAL SYS INC, MARCONI MEDICAL SYSTEMS, INC.
Application year: 1994
Main classification: G01R33
IPC codes: G01R33, A61B5